Planar waveguide-type distributed feedback laser with angular tuning

ABSTRACT

There is disclosed a distributed feedback laser made in an active thin film optical waveguide deposited on an anisotropic substrate of which the ordinary and extraordinary indices are lower than the film index. The laser is tunable by changing the direction of the pumping fringes or other spaced perturbations that produce the distributed feedback scattering with respect to the substrate optic axis. The change in direction of the pumping fringes or effective perturbation also changes the direction of propagation of laser light relative to the substrate optic axis and changes the optical fringe spacing without changing the freespace fringe spacing. Narrow-line single-mode tunable oscillation has been obtained.

United State Bjorkholm et al.

[ PLANAR WAVEGUIDE-TYPE DISTRIBUTED FEEDBACK LASER WITH ANGULAR TUNING[75] Inventors: John Ernst Bjorkholm; Charles Vernon Shank, both ofHolmdel; Thomas Patrick Sosnowski, Colts Neck, all of NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Oct. 20, 1972 21 Appl. No.: 299,257

[52] US. Cl. 331/945, 350/96 WG [51] Int. Cl. H01s 3/00 [58] Field ofSearch 331/945; 350/96 W0 [56] References Cited OTHER PUBLICATIONSBjorkholm et al., Higher Order Distributed Feedback Oscillators. Appl.Phys. Lett., Vol. 20, No. 8, (April 15, 1972) PP. 306-308.

Wang et al., Thin-Film Optical-Waveguide Mode Converters UsingGyrotropic and Anisotropic Sub- 51 Jan. 15, 1974 strates. Appl. Phys.Lett., Vol. 19, No. 6, (Sept 15, 1971) pp. 187-189.

Kogelnik et al., Coupled-Wave Theory of Distributed Feedback Lasers. J.Appl. Phys, Vol. 43. No. 5, (May 1972) PP. 2,327-2,335.

Primary Examiner-William L. Sikes Att0rney-W. L. Keefauver et al.

[57] ABSTRACT There is disclosed a distributed feedback laser made in anactive fl wn ptical waveguide deposited on an anisotropicsubstratbf'WfiiEfi the ordinary and extraordinary indices are lower thanthe film index. The laser is tunable by changing the direction of thepumping fringes or other ,spacedperturbations that produce thedistributed feedback scattering with respect to the substrate opticaxis. The change in direction of the pumping fringes or effectiveperturbation also changes the direction of propagation of laser lightrelative to the substrate optic axis and changes the optical fringespacing without changing the free-space fringe spacing. Narrow-linesingle-mode tunable oscillation has been obtained.

5 Claims, 3 Drawing Figures SUBSTRATE OPTIC AXIS RgilExlNl'gG f OUTPUTsrssasa 0km 33,1"/94.5R

PATENTEDJAK T 5 1974 saw 2 0r 2 "TM-LIKE" MODE\ MODE TE-LIKE 1 (DEGREES)EV'ENLY SPACED CIRCULAR PERTURBATIONS SUBSTRATE AXIS A ANTSOTROPICSUBSTRATE ROTATING MEANS 34 PLANAR WAVEGUIDE-TYPE DISTRIBUTED FEEDBACKLASER WITH ANGULAR TUNING BACKGROUND OF THE INVENTION This inventionrelates to the type of laser commonly known as a distributed feedbacklaser or integrated feedback laser.

Distributed feedback lasers are described, for example, in the articleTunable Distributed-Feedback Dye Laser by C. V. Shank et al., AppliedPhysics Letters, Vol. 18, page 395 (1971). A further description isfound in the article Higher-Order Distributed Feedback Oscillators by].E. Bjorkholm et al., Applied Physics Letters, Vol. 20, page 306 (1972).In a distributed feedback laser, the feedback necessary for oscillationis provided by a periodic structure which affects the propagation oflight through the active medium. Such a structure may be distributed,for example, throughout an active medium, although it could also beadjacent the medium, and takes the form of a spatial modulation of theindex of refraction of the laser medium, or its effective lightpropagation properties or its gain. The feedback mechanism is backwardBragg scattering off this periodic modulation. Because Bragg scatteringis highly frequency selective, narrow oscillation linewidths result. Thelasing wavelength is approximately equal to twice the optical length ofthe period of the modulation. If the modulation period can beconveniently changed, then tuning is readily accomplished.

One of the more versatile forms of the distributed feedback laserprovides two interfering coherent light beams incident at an angle withrespect to each other on the active medium to form the pumping fringes.A change of the relative angle will change the fringe spacing.

Nevertheless, such an adjustment of the spacing is somewhat erratic andcannot be precisely controlled because the fringe spacing tends tochange rapidly with the change in relative angle. The fringe spacing isalso subject to disturbances of any of the optical components employed.These disturbances can be minimized if the angle is kept fixed.

It is therefore an object of this invention to provide an alternativetuning mechanism for the distributed feedback laser.

SUMMARY OF THE INVENTION According to our invention, angular tuning hasbeen achieved in a distributed feedback laser constructed in a thin filmoptical waveguide arrangement in which the thin film is deposited on ananisotropic substrate of which the ordinary and extraordinary indicesare lower than the film index. The angular tuning is achieved by achange in the direction of laser oscillation in the thin film withrespect to the projection of the substrate optic axis upon the interfaceof the film in the substrate.

In a first specific embodiment, the change in the direction of laseroscillation is achieved by a change in the direction of pumping lightfringes with respect to the direction of the aforesaid projection of thesubstrate optic axis. This change is achieved typically by rotating theoptical waveguide device and the projection of the substrate optic axisrelative to the interfering pump beams that form the fringes. i

In a second specific embodiment, evenly spaced perturbations areprovided in-the optical waveguide device with circular symmetry about anaxis orthogonal to the interface of the film and the substrate. Again,the substrate optic axis has a projection upon that interface that isrotated with respect to the effective direction of pumping when thedevice is rotated about the axis of symmetry. In this case the effectivedirection of pumping is defined by supplying the pumping light in amarkedly elongated pumping area extending from edge to edge of thedevice through the axis of symmetry. This pumping area will hereinafterbe referred to as the pumping line, with reference to its elongation. Inthis sense, the pumping line has nothing to do with the pump frequency.The evenly spaced perturbations can be variations in thickness, index orgain of the optical waveguide device, and can occur either in the filmor in the portion of the substrate adjacent to the film.

With either embodiment, stable variation of the wavelength of thedistributed feedback laser may be achieved.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of ourinvention will become apparent from the following detailed descriptiontaken together with the drawing, in which:

FIG. 1 is a partially pictorial and partially block diagrammaticillustration of a preferred embodiment of our invention;

FIG. 2 shows illustrative tuning curves for the embodiments of FIGS. 1and 3; and

FIG. 3 shows another embodiment of the invention in which stablecircular perturbations in the optical waveguide device provide thedistributed feedback.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS In the embodiment of FIG. 1, itis desired to obtain a precisely tunable narrowband laser oscillationfrom a thin film optical waveguide device including the thin film 11 andthe substrate 12 on which the thin film 11 is deposited. The substrate12 is characteristically an anisotropic crystal in which the substrateoptic axis has a projection upon the interface of the film 11 and thesubstrate 12. In other words, the substrate optic axis is nonorthogonalto the interface.

Tuning is illustratively achieved in the embodiment of FIG. 1 byrotation of the just described optical waveguide device by a rotatingmeans 14 which rotates a rotatable mounting means 13 attached tosubstrate 12.

In order that the rotation not result in loss from the optical waveguidedevice, the index of the film II is higher than both the ordinary indexof refraction, n of the substrate 12 and the extraordinary index ofrefraction, n of substrate 12. In other words, the film index n, n n

Illustratively, the thin film 11 is polyurethane doped with rhodamine6G; and the substrate 12 is a crystal of ammonium dihydrogen phosphate(ADP) having a polished surface at its interface with film 11. Therefractive index, n,, of thin film 11 is 1.552.

The active medium in thin film 11 is illustratively pumped by opticalinterference fringes produced by two coherent optical beams derived fromthe single coherent pump laser source 15. Source 15 is illustratively ahigher frequency laser than that which is to be tuned in the rotatabledevice. The interfering beams are formed by focusing the single beamfrom source through spherical lens 16 and cylindrical lens 17, splittingthat beam at beamsplitter l8 and redirecting the split components fromthe adjustable reflectors l9 and 21 at a fixed angle relative to eachother. The relative path lengths can be altered as an initial oroptional adjustment. To the latter end, the optional adjusting means 20and 22 are shown for the purposes of providing the appropriatemechanical motion of reflectors 19 and 21. While this motion is notpurely translational, the degree of rotation required for any specifictranslation can be calculated from well-known formulas; and indeed, camscan be devised to coordinate the translation and rotation.

It should be understood that one of the primary advantages of thepresent invention as applied to a pumping fringe type of distributedfeedback laser is the minimizing or eliminating of the need for suchoptional adjustment of reflectors 19 and 20. For the purpose of ourconsideration hereinafter, the angle 20 shall be assumed to be constant.

Since the plane defined by the respective axes of the two interferingpump beams is stationary in spacing, the output beam from thin film 11will remain in that plane despite the rotation of thin film 11 andsubstrate 12. The utilization apparatus 23 is disposed to receive theoutput laser radiation in that plane in whatever direction it is coupledfrom thin film l 1. For ease of illustration, it is assumed that theoutput is coupled through the edge of thin film 11 in FIG. 1; but othermeans of output coupling are well known.

The pump light source 15 was illustratively a single mode ruby laserincluding a frequency doubler so that the output beam was provided at awavelength of 0.347 micrometers. Frequency doubling was accomplished ina crystal of potassium dihydrogen phosphate (KDP). The dimensions of thelong, narrow pumped region in thin film 11 were approximately 1centimeter long by 0.3 millimeters wide. Lasing occurs along the longdimension, which is also the direction of the fringe spac- In ourinitial experiments, only a small fraction of the pump light wasabsorbed in passing through the thin film 11, so that the pumping wasessentially uniform along a direction normal to the plane of the film.The output light of the waveguide laser was collected by a lens (notshown) and focused onto the entrance slit of a spectrograph, whichserved as utilization apparatus 23. This method of output coupling waschosen for our experiments since the output beam was free of angulardispersion which would complicate coupling of the light into thespectrograph. Polarizing filters (not shown) were used to study thepolarization properties of the output and thus to determine thefrequencies of the TE and TM waves for each tuning condition.

For purposes of reference, let us designate the thickness of thin film11 to be W, illustratively 0.77 micrometers, and the fringe spacing A tobe 0.1945 micrometers. For KDP the highest substrate index is n 1.524and the lowest substrate index, n;, 1.479.. We further assume that themedium overlying film 11 is air.

In a typical operation of the device of FIG. 1, the peak power output ofthe ruby laser in source 15 was about 1 megawatt; and the maximum secondharmonic power output from the KDP crystal in source 15 wasapproximately 250 kilowatts.

The particular film thickness W selected provides a single mode opticalwaveguide. The spectrum of the distributed feedback laser of FIG. 1consists of two frequencies only, one for the TE mode and one for the TMmode. Distributed feedback scattering and laser oscillation occurs nearthe Bragg condition; that is, when the wavelength in the medium is equalto 2A. The Bragg condition can be written as x 2A n.,.,, x

where A is the oscillation wavelength in air and n (A) is a functionthat depends upon the guide parameters and upon which mode is beingconsidered. In addition, H also depends upon the angle :1), which is theangle between the direction of laser oscillation in film 11 and theprojection of the substrate optic axis upon the interface of film 11 andsubstrate 12. The angle 4) lies in the plane of film l1 and cannot beseen in FIG. 1. Thus by changing q) we can change A, i.e., achievetuning.

The tuning characteristic for a typical TE mode, for

example, the TE, mode, is shown by curve 21 of FIG.

2. For a given angle 4:, which is determined by the rotation, a value ofn results as shown on the curve, and the oscillation wavelength isdetermined by Eq. (2) where ri is now a given number. Correspondingly,the tuning of the TM mode, for example, the TM,, mode, is given by curve22 of FIG. 2 which again gives a value of n, where a given angle (1) isdetermined by the rotation of the device. It will be seen that the TMmode experiences much less tuning than that of the TE mode.

It is clear that as various guide parameters, such as W, n,, n n or Achange, the dispersion curves of FIG. 2 and the operating wavelengthdetermined from Eq. (2) would also change. For the specific embodimentof FIG. 1, the perturbation spacing A is the fringe spacing and is givenby A k l2sin0,

where A, is the pump wavelength and 0 is the angle between the normal tothe film 11 and one of the interfering pump beams.

Alternatively, the necessary perturbations for distributed feedback canbe provided by means other than pump beam interference fringes, sincevariation of the parameter A is not necessary to the tuning of the laserof our invention.

For example, in the embodiment of FIG. 3 the optical waveguide deviceincludes a circularly symmetrical thin film 31 disposed on ananisotropic substrate 32, the combined device being provided with evenlyspaced circular perturbations 36 that are symmetrical about an axis ofsymmetry of film 31 and substrate 32. The evenly spaced perturbationscan be periodic variations in thickness, index of refraction, or gain ofthin film 31 or the portion of substrate 32 adjacent to thin film 31.Again, the substrate optic axis is nonorthogonal to the interface offilm 31 and substrate 32 and thus is not parallel to the axis ofsymmetry.

In this circularly symmetrical device the direction of lasing isdetermined by a pumping light line supplied from a laser source 35 andextending from edge to edge of thin film 31 through the axis ofsymmetry. Utilization apparatus 43 is aligned with the end of thispumping line. Tuning is achieved as the substrate optic axis, orspecifically its projection upon the interface of film 31 and substrate32 is rotated by a rotating means 34 with respect to the pumping line.The angle 4) is thus varied. If the film and substrate are of the samecomposition, thickness and orientation, respectively, as in FIG. 1, thetuning characteristics will be the same as shown in FIG. 2.

Rotation of the device is provided by the rotating means 34 disposedunder substrate 32 in essentially the same manner as shown in FIG. 1.The axis of rotation is the axis of symmetry of the circularlysymmetrical device of FIG. 2.

One substantial additional use of our invention other than simplyproviding a tunable oscillation is the measurement of the parameters ofunknown thin films or of optical thin films during a manufacturingprocess. For instance, if the refractive indices of the substrate andoverlying medium are known for a single mode guide, then the oscillationwavelengths of the TE, and TM, modes specify the thin film index n, andits thickness W. If n, is known in advance, then a measurement of eitherthe TE, or TM, wavelengths specifies W. At present, such measurements ofoptical thin films are made by using the prism-coupler technique, nowwell known in the art. Our technique of measuring the parameters of suchthin films provides significant advantages. For example, the distributedfeedback technique is well suited for the routine rapid processing of alarge number of thin films or waveguides, as would occur in amanufacturing process.

As an example of how the two measurement techniques are carried out, letus consider the problem of measuring n, and W for a single-mode guidefor which the refractive indices of the substrate and superstrate areboth known. For the moment, let us assume that the substrate isisotropic, to simplify the discussion. The parameters n, and W can becalculated from the equations describing the dispersion of a thin filmguide if measurements of n, for either the TB, or the TM, modes are madeat two difierent wavelengths or if the values of n, for the TE, mode andthe TM, mode are measured at the same or different wavelengths.

Thus, using the prism-coupler technique the measurement is carried outby accurately measuring the angles at which light of a given frequencyis optimally coupled into the TE, mode and the TM, mode of the guide. Ifn is the refractive index of the prism and 0,, is the internal angle ofincidence at the base of the prism for optimum coupling, then we have"eff pr w The easiest technique to measure n, using our distributedfeedback is to pump an active thin film guide with fringes having anaccurately known value of A and to measure the resulting wavelength ofthe TE, mode and of the TM, mode. For our particular setup, Eq. (3)yields n (MM) sin0,

where it is necessary that sin0 be accurately known. A conventionalangle measurement could be utilized to determine 0. However, sin0 may bemore conveniently determined to the desired accuracy by measuring thewavelength of oscillation in an organic dye solution having anaccurately known index-of-refraction and pumped with the sameexperimental setup.

Advantages of the distributed feedback technique over the prism-couplertechnique are as follows. First, no contact is made to the guide beingmeasured. Physical distortion of the guide is possible using a prismcoupler; and the degree of coupling affects the propagationcharacteristics of the film. Secondly, it is more convenient to makemeasurements of wavelengths to the desired accuracy. Thirdly, themanipulation required in the prism-coupler technique has been done awaywith. Finally, all the data needed to determine n; and W can be obtainedin a single shot. The distributed feedback method is very rapid once theapparatus is set up and the angle 0 accurately measured. The majordisadvantage is that as outlined above it is useful only with activeoptical waveguides. It is hoped that the technique can also be used withpassive guides by the technique of evanescent field coupling to adjacentactive medium.

For an anisotropic substrate, a single-mode film is the simplest case.The plane defined by the intersecting axes of the interfering pump beamswould be oriented at 0 or with respect to the substrate optic axis. Thevalues for n, for each mode, TM, and TE,, are then those at theappropriate extremities of curves 211 and 22 of FIG. 2.

In all cases it is to be understood that the abovedescribed arrangementsare merely illustrative of a small number of the many possibleapplications of the principles of the invention. Numerous and variedother arrangements in accordance with these principles may readily bedevised by those skilled in the art without departing from the spiritand scope of the invention.

We claim:

1. A laser comprising an anisotropic substrate having an optic axisnon-orthogonal to a major surface thereof, a waveguiding film adjoiningsaid major surface and having an index of refraction higher than bothindices of refraction of said substrate, one of said film and substrateincluding an active medium, and means for pumping said medium toestablish a population inversion between energy levels in said mediumwith optical radiation in a directionally-oriented pattern determiningan effective direction of distributed feedback scattering, said patternof pumping radiation and the combination of said substrate and saidfilms together providing periodic optical perturbations to yield thedistributed feedback scattering, said laser including means for tuningits frequency of oscillation by changing the angle d: between theeffective direction of feedback scattering and the projection of saidoptic axis on said major surface.

2. A laser according to claim 1 in which the means for pumping themedium includes means for supplying to said medium two coherentinterfering pumping beams yielding interference fringes that yield alongtheir minimum spacing distributed feedback scattering and in which themeans for tuning by changing the angle (1) comprise means for changingthe orientation of the fringes with respect to the projection of thesubstrate optic axis, comprising means for rotating the waveguiding filmin a plane effective to change said angle 4).

3. A laser according to claim 1 in which the combination of theanisotropic substrate and the waveguiding film has an axis ofsubstantially circular symmetry and includes evenly spaced perturbationsin material properties of said combination, said perturbations beingcircularly symmetrical about said axis of symmetry, and the means forpumping the medium comprises means for supplying to said combinationcoherent pumping light in an elongated continous pattern illuminating anelongated region extending through the axis of symmetry between oppositeedges of the film, the means for tuning the frequency by changing theangle (1) comprising means for rotating the combination with respect tothe elongated region.

4. A laser according to claim 3 in which the rotating means rotates thecombination about the axis of symmetry.

5. An optical thin film waveguide device comprising an anisotropicsubstrate having an optic axis substantially parallel a major surfacethereof, a waveguiding film adjoining said major surface and having anindexof-refraction higher than both indices of refraction of thesubstrate, one of said film and substrate including an active medium,and means for pumping said medium with optical radiation interferencefringes, said optical thin film waveguide device including means fortuning its frequency of oscillation in that said pumping means includesmeans for changing the orientation of the fringes with respect to saidoptic axis.

1. A laser comprising an anisotropic substrate having an optic axisnon-orthogonal to a major surface thereof, a waveguiding film adjoiningsaid major surface and having an index of refraction higher than bothindices of refraction of said substrate, one of said film and substrateincluding an active medium, and means for pumping said medium toestablish a population inversion between energy levels in said mediumwith optical radiation in a directionally-oriented pattern determiningan effective direction of distributed feedback scattering, said patternof pumping radiation and the combination of said substrate and saidfilms together providing periodic optical perturbations to yield thedistributed feedback scattering, said laser including means for tuningits frequency of oscillation by changing the angle phi between theeffective direction of feedback scattering and the projection of saidoptic axis on said major surface.
 2. A laser according to claim 1 inwhich the means for pumping the medium includes means for supplying tosaid medium two coherent interfering pumping beams yielding interferencefringes that yield along their minimum spacing distributed feedbackscattering and in which the means for tuning by changing the angle phicomprise means for changing the orientation of the fringes with respectto the projection of the substrate optic axis, comprising means forrotating the waveguiding film in a plane effective to change said anglephi .
 3. A laser according to claim 1 in which the combination of theanisotropic substrate and the waveguiding film has an axis ofsubstantially circular symmetry and includes evenly spaced perturbationsin material properties of said combination, said perturbations beingcircularly symmetrical about said axis of symmetry, and the means forpumping the medium comprises means for supplying to said combinationcoherent pumping light in an elongated continous pattern illuminating anelongated region extending through the axis of symmetry between oppositeedges of the film, the means for tuning the frequency by changing theangle phi comprising means for rotating the combination with respect tothe elongated region.
 4. A laser according to claim 3 in which therotating means rotates the combination about the axis of symmetry.
 5. Anoptical thin film waveguide device comprising an anisotropic substratehaving an optic axis substantially parallel a major surface thereof, awaveguiding film adjoining said major surface and having anindex-of-refraction higher than both indices of refraction of thesubstrate, one of said film and substrate including an active medium,and means for pumping said medium with optical radiation interferencefringes, said optical thin film waveguide device including means fortuning its frequency of oscillation in that said pumping means includesmeans for changing the orientation of the fringes with respect to saidoptic axis.